(1) Technical Field
This invention relates generally to electromagnetic compatibility (EMC) and more specifically to the construction of a low cost, portable, inflatable and modular enclosure designed to shield against electromagnetic interference (EMI) while conducting EMC/EMI testing.
(2) Description of the Prior Art
The following four documents relate to a method of and apparatus for an electromagnetically anechoic chamber and shield structure.
U.S. Pat. No. 3,955,199 issued May 4, 1976 to Robert W. Hutzelman and Everitt E. Fairbanks describes a method of and apparatus for ground testing doppler navigation sets. The device is an improved doppler microwave radar simulator which simulates a moving earth, without moving components, so as to minimize weight, complexity and size.
U.S. Pat. No. 5,134,405 issued Jul. 28, 1992 to Katsuo Ishihara, Katano and Yoshio Tomiyama, Kawanishi shows a design for a small-sized (7 m.times.3 m.times.2.5 m in height) electromagnetically anechoic chamber for EMC (Electromagnetic Compatibility) tests of electronic devices.
U.S. Pat. No. 5,534,873 issued Jul. 9, 1996 to Alan R. Wcichman, Barton G. Ferrell et al. describes a method for determining the far field radar signature of relatively large and complex objects such as fighter aircraft.
PCT/US93/00914, WO 93/15293, issued Aug. 5, 1993 to Paes, Ned Z.; Phillips, Terry; Thomas, Robert E.; and Aldridge, Joseph, H., Bulwark Electromagnetic Material Applications Inc. (BEMA) [US/US] describes a structurally self-contained RF-shielded enclosure which is rapidly deployable due to its ultra light weight and case of assembly.
Generally, components and assemblies containing electrical devices are subject to EMC tests by their manufacturer. Two conventional techniques used for EMC testing are accomplished in an open-field test site and in electromagnetic anechoic enclosures. The open-field test site is usually done in open air away from populated areas where energy levels of noises are high. Briefly, the arrangement of an open-field test site includes a platform to support a device to be tested, a post for mounting an antenna which is provided at a distant position from the platform in a manner to slide up and down on the post and a measuring instrument which is connected by an electric cable to the antenna. In making noise assessment of radiating electromagnetic waves, the wave emanating from the electrical assembly is measured with the antenna that receives the radiated wave to determine whether the data is below a specified point. When making test irradiation in a strong electric field, a strong electric field is irradiated from the antenna to the device to be tested, and the device to be tested is measured to detect its abnormal state. A conventional anechoic enclosure is surrounded by iron sheets to shield against electromagnetic waves in the radio frequency range. The interior walls and ceiling are lined with microwave absorbent materials. EMC tests can also be made in an anechoic enclosure using the same type of testing apparatus as described for the open-space test site.
When dealing with large assemblies such a military vehicles, aircraft, etc., each of the electrical devices contained within these large assemblies is subject to EMC testing. The amount of testing is enormous in terms of scheduled hours and manpower. Open-air testing is an option for large assemblies. By definition, an open-field test site for EMC testing is a noiseless field. Testing cannot be done in municipalities where electromagnetic noise levels are notably high. If an open-field testing method is used, it must be done in remote areas; however, this is also difficult as data is usually masked by high power transmitters. Open-field testing is also time-consuming since reliable test results are dependent on the stability of localized atmospheric conditions, i.e., temperature and humidity. Testing cannot be done in rain, snow or strong winds.
To overcome these difficulties, tests must be conducted in a large anechoic or shielded chamber, a costly option. Besides the high cost of such structures, such a facility requires a permanent shelter and storage area. The alternative is to devise a means of building a shield facility large enough to accommodate an aircraft but with materials and construction techniques that will render the structure low cost, easy and fast to assemble and dissemble (under one hour), portable, semi-permanent and effective.
FIG. 1 is a schematic drawing of the prior art, showing a conventional, structurally self-contained RF-shielded enclosure which is rapidly deployable due to its light weight and easy assembly. The enclosure can be assembled to create a free-standing room within an existing non-secure room or environment.
FIG. 1 illustrates the conventional construction and its limitations: lightweight metallized fabric 102 is sewn together with rugged tent fabric 101 and ballistics cloth 103. Fiberglass rods 21c, 22a, 23a, 24 are connected to aluminum sleeves and unions 21a, 22b, 23g to configure the room. A bulkhead assembly 306 penetrates the side wall and provides filtered power and electronic signal interfaces. Honeycomb air guides 305 facilitate air exchange within the room.